1. Field of the Invention
The present invention relates to a transmission type liquid crystal display device which includes switching elements such as thin film transistors (hereinafter, referred to as xe2x80x9cTFTsxe2x80x9d) as addressing elements and is used for displays of computers, TV sets, and the like, and a method for fabricating such a transmission type liquid crystal display device.
2. Description of the Related Art
FIG. 16 is a circuit diagram of a conventional transmission type liquid crystal display device provided with an active matrix substrate.
Referring to FIG. 16, the active matrix substrate includes a plurality of pixel electrodes 1 arranged in a matrix and TFTs 2 used as switching elements connected to the respective pixel electrodes 1. Gate electrodes of the TFTs 2 are connected to gate lines 3 for supplying a scanning (gate) signal, so that the gate signal can be input into the gate electrodes to control the driving of the TFTs 2. Source electrodes of the TFTs 2 are connected to source lines 4 for supplying an image (data) signal, so that the data signal can be input into the corresponding pixel electrodes 1 via the TFTs when the TFTs are being driven. The gate lines 3 and the source lines 4 run adjacent to the pixel electrodes 1 and are arranged in a matrix to cross each other. Drain electrodes of the TFTS 2 are connected to the respective pixel electrodes 1 and storage capacitors 5. Counter electrodes of the storage capacitors 5 are connected to common lines 6. The storage capacitor 5 is used for holding a voltage applied to a liquid crystal layer. The storage capacitor is provided in parallel to a liquid crystal capacitor which includes the liquid crystal layer sandwiched between a pixel electrode provided on an active matrix substrate and a counter electrode provided on a counter substrate.
FIG. 17 is a sectional view of a one-TFT portion of the active matrix substrate of the conventional liquid crystal display device.
Referring to FIG. 17, a gate electrode 12 connected to the gate line 3 shown in FIG. 16 is formed on a transparent insulating substrate 11. A gate insulating film 13 is formed covering the gate electrode 12. A semiconductor layer 14 is formed on the gate insulating film 13 so as to overlap the gate electrode 12 via the gate insulating film 13, and a channel protection layer 15 is formed on the center of the semiconductor layer 14. n+-Si layers as a source electrode 16a and a drain electrode 16b are formed covering the end portions of the channel protection layer 15 and portions of the semiconductor layer 14, so that they are separated from each other at the top of the channel protection layer 15. A metal layer 17a which is to be the source line 4 shown in FIG. 16 is formed to overlap the source electrode 16a as one of the n+-Si layers. A metal layer 17b is formed to overlap the drain electrode 16b as the other n+-Si layer so as to connect the drain electrode 16b and the pixel electrode 1. An interlayer insulating film 18 is formed covering the TFT 2, the gate line 3, and the source line 4.
A transparent conductive film is formed on the interlayer insulating film 18 to constitute the pixel electrode 1. The transparent conductive film is connected to the metal layer 17b which is in contact with the drain electrode 16b of the TFT 2 via a contact hole 19 formed through the interlayer insulating film 18.
Thus, since the interlayer insulating film 18 is formed between the pixel electrode 1 and the underlying layers including the gate and source lines 3 and 4, it is possible to overlap the pixel electrode 1 with the lines 3 and 4. Such a structure is disclosed in Japanese Laid-Open Patent Publication No. 58-172685, for example. With this structure, the aperture ratio improves and, since the electric field generated by the lines 3 and 4 is shielded, the occurrence of disclination can be minimized.
Conventionally, the interlayer insulating film 18 is formed by depositing an inorganic material such as silicon nitride (SiN) to a thickness of about 500 nm by chemical vapor deposition (CVD).
The above conventional liquid crystal display device has disadvantages as follows.
When a transparent insulating film made of SiNx, SiO2, TaOx, and the like is formed on the interlayer insulating film 18 by CVD or sputtering, the surface of the film directly reflects the surface profile of the underlying film, i.e., the interlayer insulating film 18. Therefore, when the pixel electrode 1 is formed on the transparent insulating film, steps will be formed on the pixel electrode 1 if the underlying film has steps, causing disturbance in the orientation of liquid crystal molecules. Alternatively, the interlayer insulating film 18 may be formed by applying an organic material such as polyimide to obtain a flat pixel portion. In such a case, however, in order to form the contact holes for electrically connecting the pixel electrodes and the drain electrodes, a series of steps including photo-patterning using a photoresist as a mask, etching for forming the contact holes, and removal of the photoresist are required. A photosensitive polyimide film may be used to shorten the etching and removal steps. In this case, however, the resultant interlayer insulating film 18 appears colored. This is not suitable for a liquid crystal display device requiring high light transmission and transparency.
The other disadvantage is as follows. When the pixel electrode 1 overlaps the gate line 3 and the source line 4 via the interlayer insulating film 18, the capacitances between the pixel electrode 1 and the gate line 3 and between the pixel electrode 1 and the source line 4 increase. In particular, when an inorganic film made of silicon nitride and the like is used as the interlayer insulating film 18, the dielectric constant of such a material is as high as 8 and, since the film is formed by CVD, the thickness of the resultant film is as small as about 500 nm. With such a thin interlayer insulating film, the capacitances between the pixel electrode 1 and the lines 3 and 4 are large. This causes the following problems (1) and (2). Incidentally, in order to obtain a thicker inorganic film made of silicon nitride and the like, an undesirably long time is required in the aspect of the fabrication process.
(1) When the pixel electrode 1 overlaps the source line 4, the capacitance between the pixel electrode 1 and the source line 4 becomes large. This increases the signal transmittance, and thus a data signal held in the pixel electrode 1 during a holding period fluctuates depending on the potential thereof. As a result, the effective voltage applied to the liquid crystal in the pixel varies, causing, in particular, vertical crosstalk toward a pixel adjacent in the vertical direction in the actual display.
In order to reduce the influence of the capacitance between the pixel electrode 1 and the source line 4 appearing on the display, Japanese Laid-Open Patent Publication No. 6-230422 proposes a driving method where the polarity of a data signal to be supplied to the pixels is inverted every source line. This driving method is effective for a black-and-white display panel where the displays (i.e., data signals) of adjacent pixels are highly correlated with each other. However, it is not effective for a color display panel for normal notebook type personal computers and the like where pixel electrodes are arranged in a vertical stripe shape (in color display, a square pixel is divided into three vertically long rectangular picture elements representing R, G, and B, forming a vertical stripe shape). The display color of pixels connected to one source line is different from that of pixels connected to an adjacent source line. Accordingly, the proposed driving method of inverting the polarity of the data signal every source line is not effective in reducing crosstalk for the general color display, though it is effective for the black-and-white display.
(2) When the pixel electrode 1 overlaps the gate line 3 for driving the pixel, the capacitance between the pixel electrode 1 and the gate line 3 becomes large, increasing the feedthrough of the write voltage to the pixel due to a switching signal for controlling the TFT 2.
The transmission type liquid crystal display device of this invention includes: gate lines; source lines; and switching elements each arranged near a crossing of each gate line and each source line. A gate electrode of each switching element is connected to the gate line, a source electrode of the switching element is connected to the source line, and a drain electrode of the switching element is connected to a pixel electrode for applying a voltage to a liquid crystal layer, wherein an interlayer insulating film formed of an organic film with high transparency is provided above the switching element, the gate line, and the source line. The pixel electrode, formed of a transparent conductive film, is provided on the interlayer insulating film.
In one embodiment of the invention, the device further includes a connecting electrode for connecting the pixel electrode and the drain electrode, wherein the interlayer insulating film is provided above the switching element, the gate line, the source line, and the connecting electrode. The pixel electrode is formed on the interlayer insulating film so as to overlap at least the gate line or the source line at least partially, and the connecting electrode and the pixel electrode are connected with each other via a contact hole formed through the interlayer insulating film.
In one embodiment of the invention, the interlayer insulating film is made of a photosensitive acrylic resin.
In one embodiment of the invention, the interlayer insulating film is made of a resin which is made transparent by optical or chemical decoloring treatment.
In one embodiment of the invention, the pixel electrode and at least one of the source line and the gate line overlap each other by 1 xcexcm or more in a line width direction.
In one embodiment of the invention, the thickness of the interlayer insulating film is 1.5 xcexcm or more.
In one embodiment of the invention, the connecting electrode is formed of a transparent conductive film.
In one embodiment of the invention, the device further includes a storage capacitor for holding a voltage applied to the liquid crystal layer, wherein the contact hole is formed above either an electrode of the storage capacitor or the gate line.
In one embodiment of the invention, a metal nitride layer is formed below the contact hole to connect the connecting electrode and the pixel electrode.
In one embodiment of the invention, the device further includes a storage capacitor for holding a voltage applied to the liquid crystal layer, wherein a capacitance ratio represented by expression (1):
Capacitance ratio=Csd/(Csd+Cls+Cs)xe2x80x83xe2x80x83(1)
is 10% or less, wherein Csd denotes a capacitance value between the pixel electrode and the source line, Cls denotes a capacitance value of a liquid crystal portion corresponding to each pixel in an intermediate display state, and Cs denotes a capacitance value of the storage capacitor of each pixel.
In one embodiment of the invention, the shape of the pixel electrode is rectangular with a side parallel to the gate line being longer than a side parallel to the source line.
In one embodiment of the invention, the device further includes a driving circuit for supplying to the source line a data signal of which polarity is inverted for every horizontal scanning period, and the data signal is supplied to the pixel electrode via the switching element.
In one embodiment of the invention, the device further includes a storage capacitor for maintaining a voltage applied to the liquid crystal layer, the storage capacitor including: a storage capacitor electrode; a storage capacitor counter electrode; and an insulating film therebetween; wherein the storage capacitor electrode is formed in the same layer as either the source line or the connecting electrode.
In one embodiment of the invention, the storage capacitor counter electrode is formed of a part of the gate line.
In one embodiment of the invention, the pixel electrode and the storage capacitor electrode are connected via the contact hole formed above the storage capacitor electrode.
In one embodiment of the invention, the contact hole is formed above either the storage capacitor counter electrode or the gate line.
In one embodiment of the invention, the interlayer insulating film is formed of a photosensitive resin containing a photosensitive agent which has a reactive peak at the i line (365 nm).
According to another aspect of the invention, a method for fabricating a transmission type liquid crystal display device is provided. The method includes the steps of: forming a plurality of switching elements in a matrix on a substrate; forming a gate line connected to a gate electrode of each switching element and a source line connected to a source electrode of the switching element, the gate line and the source line crossing each other; and forming a connecting electrode formed of a transparent conductive film connected to a source electrode of the switching element. The method further includes forming an organic film with high transparency above the switching elements, the gate lines, the source lines, and the connecting lines by a coating method and patterning the organic film to form an interlayer insulating film and contact holes through the interlayer insulating film to reach the connecting electrodes. The method also includes the step of forming pixel electrodes formed of transparent conductive films on the interlayer insulating film and inside the contact holes so that each pixel electrode overlaps at least either the gate line or the source line at least partially.
In one embodiment of the invention, the patterning of the organic film is conducted by either one of the following steps: exposing the organic film to light and developing the exposed organic film, or etching the organic film by using a photoresist on the organic film as an etching mask.
In one embodiment of the invention, the patterning of the organic film includes the steps of: forming a photoresist layer containing silicon on the organic film; patterning the photoresist layer; and etching the organic film by u sing the pattern ed photoresist layer as an etching mask.
In one embodiment of the invention, the patterning of the organic film includes the steps of: forming a photoresist layer on the organic film; coating a silane coupling agent on the photoresist layer and oxidizing the coupling agent; patterning the photoresist layer; and etching the organic film by using the patterned photoresist layer covered with the oxidized coupling agent as an etching mask.
In one embodiment of the invention, the etching step is a step of dry etching using an etching gas containing at least one of CF4, CF3H and SF6.
In one embodiment of the invention, the organic film is formed by using a photosensitive transparent acrylic resin which dissolves in a developing solution when exposed to light, and the interlayer insulating film and the contact holes are formed by exposing the photosensitive transparent acrylic resin to light and developing the photosensitive transparent acrylic resin.
In one embodiment of the invention, the method further includes the step of, after the light exposure and development of the organic film, exposing the entire substrate to light for reacting a photosensitive agent contained in the photosensitive transparent acrylic resin, thereby decoloring the photosensitive transparent acrylic resin.
In one embodiment of the invention, a base polymer of the photosensitive transparent acrylic resin includes a copolymer having methacrylic acid and glycidyl methacrylate and the photosensitive transparent acrylic resin contains a quinonediazide positive-type photosensitive agent.
In one embodiment of the invention, the photosensitive transparent acrylic resin for forming the interlayer insulating film has a light transmittance of 90% or more for light with a wavelength in the range of about 400 nm to about 800 nm.
In one embodiment of the invention, the organic film has a thickness of about 1.5 xcexcm or more.
In one embodiment of the invention, the method further includes the step of, before the formation of the organic film, irradiating with ultraviolet light a surface of the substrate where the organic film is to be formed.
In one embodiment of the invention, the method further includes the step of, before the formation of the organic film, applying a silane coupling agent on a surface of the substrate where the organic film is to be formed.
In one embodiment of the invention, the material for forming the organic film contains a silane coupling agent.
In one embodiment of the invention, the silane coupling agent includes at least one of hexamethyl disilazane, dimethyl diethoxy silane, and n-buthyl trimethoxy.
In one embodiment of the invention, the method further includes the step of, before the formation of the pixel electrode, ashing the surface of the interlayer insulating film by an oxygen plasma.
In one embodiment of the invention, the ashing step is conducted after the formation of the contact holes.
In one embodiment of the invention, the interlayer insulating film includes a thermally curable material and the interlayer insulating film is cured before the ashing step.
In one embodiment of the invention, the thickness of the ashed portion of the interlayer insulating film is in the range of about 100 to 500 nm.
In one embodiment of the invention, the thickness of the pixel electrode is about 50 nm or more.
In one embodiment of the invention, the interlayer insulating film is formed by developing the photosensitive transparent acrylic resin with tetramethyl ammonium hydroxyoxide developing solution with a concentration of about 0.1 mol % to about 1.0 mol %.
In one embodiment of the invention, the method further includes the step of, after the formation of the contact holes through the interlayer insulating film, decoloring the interlayer insulating film by irradiating the interlayer insulating film with ultra-violet light.
In one embodiment of the invention, the method further includes the step of, before the formation of the organic film, forming a silicon nitride film on a surface of the substrate where the organic film is to be formed.
Thus, the invention described herein makes possible the advantage of (1) providing a transmission type liquid crystal display device where flat pixel electrodes overlap respective lines to improve the aperture ratio of the liquid crystal display, minimize disturbance in the orientation of liquid crystal molecules, and simplify the fabrication process. Furthermore, and the influence of the capacitance between the pixel electrodes and the lines appearing on the display, such as crosstalk, can be reduced to achieve a good display. The invention described herein also makes possible the advantage of (2) providing a method for fabricating such a transmission type liquid crystal display device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.